A semiconductor device may include an integrated circuit including metal-oxide-semiconductor field-effect transistors (MOS-FETs). With an increasing integration density of the semiconductor device, MOS-FETs are being scaled down, and this leads to deterioration in operational characteristics of the semiconductor device. Furthermore, it is becoming harder to achieve transistor performance required by a user. Various FET structures have been suggested to overcome these technical issues. For example, a high-k metal gate structure has been proposed to replace a conventional FET, in which a silicon oxide layer and a polysilicon layer are used for a gate insulating layer and a gate-electrode, respectively.